In the data capture field, there are many applications where hand-held data terminals should be of rugged construction so as to survive rough handling. Many operators are not inclined toward painstaking or precise manipulations. An example is in the use of RF data capture terminals on forklift trucks in factories and warehouses where items to be transported are identified by bar codes. Other examples are found in the fields of route delivery and direct store delivery where many items are handled and the terminal means automates the accounting function. Even in applications where bar code data is transmitted on-line to a central station, it may be desirable for hand-held terminals to be inserted into docking apparatus for the interchange of data signals, e.g., the loading of scheduling information or the like into the terminal at the beginning of a working shift. Further, where terminal means has memory capacity for accumulating data during a delivery operation or the like, it may be desirable for such data to be transferred to a printer so that a hard copy may be produced. In cases where rechargeable batteries are used, the docking apparatus may provide for the recharging of such batteries at the same time as data communication is taking place.
It is conceived that it would be highly advantageous to provide a data capture system with docking apparatus adaptable to a wide range of terminal means, and which furthermore could be quickly and simply loaded in a relatively foolproof manner, and without requiring attention and care from operators engaged in physically demanding and arduous work routines. A docking apparatus would be desirable that completely avoids the use of mating pin and socket type electrical connections, and that does not rely on a specialized configuration of the terminal, e.g., the provision of an optical scanner tip which may be used for data communication. However, pin and socket type connectors may be utilized.
In connection with the use of portable data systems it is conceived that it would be highly advantageous to be able to readily upgrade a basic hand-held terminal to incorporate bar code scan type readers and various image readers as they are progressively improved and developed. A particular goal would be the implementation of the auxiliary image reader function in a rugged configuration free of moving parts. However, in the case of autofocus readers, the current state of the art may require dynamic components for the sake of optimum compactness and economy.
Optical data file readers taught by the prior art often employed analog circuitry and analog signal processing to accomplish digitization of data file images. For example, Dodson, III, U.S. Pat. No. 3,892,950, discloses a circuit for detecting transitions in an alternating signal representing binary information. Dodson's invention is essentially a 1-bit A/D (analog-to-digital) convertor having two output states, a high or low binary output voltage. Dodson's invention uses entirely analog circuitry to calculate the mean value of the input data file signal by analog summation of the input signal and a delayed version of the input signal. The output of the analog summing circuitry is then divided by two using analog circuitry to arrive at the mean value of the input signal, which was used as a digitization threshold. A comparator circuit detected when the delayed signal crossed the threshold value and delivered the digital output signal: either a high voltage or a low voltage.
Other prior art optical data file readers also employed analog circuitry and analog signal processing to accomplish digitization of the data file signal along the same lines as the Dodson invention. In one such invention, Coles, Jr., U.S. Pat. No. 3,751,636, operated on the same principal as Dodson, except it used analog circuitry to find the peak value of the input signal, which was divided by two to arrive at the digitization threshold. As was the case with Dodson, Coles invention is essentially a 1-bit A/D converter wherein all of the mathematical computations are preformed entirely in the analog signal processing domain.
State of the art signal processing employs digital signal processing rather than analog signal processing. A discussion of the advantages of digital signal processing over analog signal processing is given in Digital Signal Processing, by John G. Proakis and Dimitris G. Manolakis, 1988.
There are many reasons why digital signal processing may be preferable to processing the signal directly in the analog domain . . . A digital programmable system allows flexibility in reconfiguring the digital signal processing operations simply by changing the program. Reconfiguration of an analog system usually implies a redesign of the hardware, testing, and verification that it operates properly e.g. changing form a Coles type implementation to a Dodson type implementation!. . . Digital signal processing provides better control of accuracy requirements. Tolerances in analog circuit components make it extremely difficult for the system designer to control the accuracy of an analog signal processing system. On the other hand, a digital system provides much better control of accuracy requirements (p.5).
The digital signal processing method allows for the implementation of more sophisticated signal processing algorithms. It is usually difficult to perform precise mathematical operations on signals in analog form.
However, these operations can be routinely implemented on a digital computer or data terminal! by means of software (p.5).
Digital signal processing provides an alternative method for processing the analog signal . . . In order to perform the processing digitally, there is a need for an interface between the analog signal and the digital processor. This interface is called an analog-to-digital (A/D) convertor. The output of the A/D converter is a digital signal that is appropriate as an input to the digital processor (p.4)
As stated in The Electrical Engineering Handbook, "the major factors that determine performance of D/A and A/D convertors are resolution, sampling rate, speed, and linearity." (Richard C. Dorf, The Electrical Engineering Handbook, p.771, 1991). Further, "in an A/D system, the resolution is the smallest change in voltage that can be detected by the system and that can produce a change in the digital code. The resolution determines the number of digital codes, or quantization levels, that will be recognized or produced by the circuit." (id., pp.771-772).
Typical digital signal processing systems employ 8-bit A/D convertors. Using an 8-bit AND convertor yields 2.sup.8 =256 levels of amplitude quantization. Additionally, the bandwidth, or sampling rate, of the signal processors determines the number of samples of a signal that may be taken per unit time. An effective sampling rate of 5 MHz would result in 200 ns between samples. Thus an analog input signal may be converted to a discrete-time 8-bit digital signal having 256 levels of amplitude quantization wherein one sample of the input signal is taken every 200 ns.
A high performance digital signal processor (DSP) such as a Texas Instruments TMS320C51 is capable of performing mathematical computations too complex to implement with analog circuitry at high operational bandwidths. Numerous mathematical algorithms may be implemented using a DSP, and if necessary the algorithms may easily be changed by merely changing the software. The DSP implemented mathematical algorithms may be computationally more complex, more accurate, and hence more reliable than analog implemented algorithms by several orders of magnitude
Thus a basic hand-held data terminal may employ a multitude of interchangeable modules each performing a unique data capturing function. Using digital signal processing technology allows for flexibility and ease of adaptation of the data terminal to an intended use in combination with a particular module. The applications of each module may include intensive digital signal processing in conjunction with a powerful digital signal coprocessor. For example a hand-held data terminal may include a bar code type optical image reader employing digital signal processing techniques to attain such functions as autofocusing by using complex mathematical algorithms.